This invention relates to a method of making an ion filter of an electrically insulating material such as glass, ceramic or the like which has metallized surfaces of approximately hyperbolic cross-sectional shape for generating the necessary electric fields in an ion filter. The invention further relates to an ion filter made with the method.
German Pat. No. 944,900 discloses the making of a four-pole ion filter from four metal elements such as metal sheet members or metal rods. It is a disadvantage of the manufacturing method disclosed in this German patent that an adjustment of four individual elements, the relative position of which has to be extremely precise particularly as concerns their parallelism, involves substantial difficulties. Particularly as a result of the high temperature differences (up to 450.degree. C.) appearing during the heating of a mass analyzer, the metal components often undergo deformations because of their relatively high coefficient of expansion. Consequently, the generation of electric fields having a configuration sufficiently exact for mass analysis is no longer possible after such deformations have occurred.
Because of the above-outlined disadvantages, more recently ion filters have been provided which are formed of four ceramic rods of circular cross section and carrying a metal coating thereon. In this connection reference is made to British Pat. No. 1,263,762, as well as U.S. Pat. Nos. 3,197,633 and 3,553,451. Here too, the rods, according to these methods, have to be arranged in a precise relationship with respect to one another in a circumstantial and thus expensive manner involving, for example, ceramic holder arrangements. Particularly, because of the expensive adjusting steps, assembling the ion filter is still wrought with difficulties and involves substantial expense.
In order to overcome the difficulties experienced during the assembly of the ion filters and to obtain an ion filter which operates well even under high temperatures, it has been further proposed to manufacture the ion filter as a single body made of ceramic, glass or similar material. Thus, German Pat. No. 1,297,360 discloses the application of glass on a core body. In German Laid-Open Application (Offenlegungsschrift) No. 1,773,194 it is disclosed to provide a cylindrical ceramic body havng, in its interior, electrically conducting layers for generating the required electric fields. Further, German Laid-Open Application (Offenlegungsschrift) No. 2,215,763 teaches the making of a one-piece ion filter by extruding a ceramic body which is subsequently pressed and fired, and then the inner faces of the body are metallized.
The significant disadvantage of the above-outlined manufacturing methods resides in the fact that the surfaces to be metallized are located in the interior of a generally tubular body and are therefore accessible only with difficulty. Both the forming of the surfaces as well as their metallization is therefore difficult to perform and, as a result, irregularities in the surfaces cannot be avoided. This then leads to non-uniform electric fields. The length of the ion filters manufactured in the above-outlined manner is thus limited; this is disadvantageous, particularly for high-energy ions. It is possible, to be sure, to arrange two or more ion filters in series; this, however, again involves the above-described disadvantages regarding the expensive adjustment steps. In the method disclosed in German Laid-Open Application (Offenlegungsschrift) No. 2,215,763 the length of the ion filter is further limited in view of the particular manner in which the filter is manufactured. The hyperbolic inner faces of the ion filters obtained by extruding a soft ceramic mass must retain their exact form during the firing step as well; the longer the ceramic body, the more difficult it is to meet this requirement. In an ion filter manufactured in this manner, because of the relatively narrow axial passage and the complicated hyperbolic form of the inner faces, a post-treatment of these surfaces to ensure satisfactory surface properties is possible only --if at all--with prohibitive cost. Further, a uniform metallization of these surfaces also involves significant difficulties.